Comment by Avshalom

https://github.com/nornagon/jonesforth/blob/master/jonesfort...

This is (the now pedagogically famous) JonesForth. It's ~2000 lines of HEAVILY commented assembly, and at then you have everything you need to start writing a fully functional Forth+standard library (which is done in jonesforth.f in a ~1800 lines of heavily commented forth). That's not even as tiny as forth can be, and it gives you a lowish level language that is as/more modifiable and extensible as any Lisp.

There is basically no faster way to get from bare metal to a comfortable humane and interactive (so you can write more stuff) computing environment than writing a forth.